Passive heat removal system for nuclear reactors

ABSTRACT

A nuclear reactor is configured with an intermediate coolant loop for transferring thermal energy from the reactor core for a useful purpose. The intermediate coolant loop includes a bypass flowpath with an air heat exchanger for dumping reactor heat during startup and/or shutdown. A fluidic diode along the bypass flowpath asymmetrically restricts flow across the bypass flowpath, inhibiting flow in a first flow direction during a full power operating condition and allowing a relatively uninhibited flow in a second direction during a startup and/or shut down low power operating condition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 63/066,785, filed Aug. 17, 2020, entitled “CARTRIDGECORE BARREL FOR NUCLEAR REACTOR,” the contents of which is incorporatedherein by reference in its entirety.

BACKGROUND

Most nuclear reactors have a core within which fuel elements and controlelements are supported in different interrelated arrangements to supporta critical reactivity to control the output of the reactor. Coolant istypically forced through passages between fuel elements and controlelements to transfer heat generated by fissioning fuel elements to aheat exchanger to be used for useful purposes.

When a reactor is shut down, such as by inserting control elements intothe reactor core to reduce the reactivity, the fission products continueto generate heat as they experience radioactive decay. In some cases,the decay heat power level is initially about 6-7% of the core outputpower prior to shutdown, which can result is a significant amount ofheat that must be dealt with upon reactor shutdown.

A residual heat removal system is a standard safety system designed todeal with decay heat from a reactor that has shut down, and may includeemergency generators to drive circulation pumps, passive heat removalsystems, and other type of heat removal systems. Without a properfunctioning decay heat removal system, the residual heat can causecatastrophic failures within the reactor systems.

Some reactors utilize a direct reactor auxiliary cooling system (DRACS)or a reactor vessel auxiliary cooling system (RVACS), to deal withresidual heat and these systems are primarily safety systems relied uponfor dumping decay heat after a reactor shutdown.

It would be an improvement in the art to use a passive heat removalsystem that not only aids in removing residual decay heat in a passivesystem, but also functions to remove heat from the reactor duringstartup procedures. It would be a further advantage if such a system wasnot classified as a safety system and therefore had the requirement tobe designed and built to safety grade standards, but rather, was acommercial first line of defense in addition to such a safety system.

SUMMARY

According to some embodiments, a passive heat removal system functionsto allow decay heat removal, both during startup and during shutdown ofa nuclear reactor by allowing both forced and natural circulationbetween a hot leg and a cold leg of a heat exchanger situated within theintermediate fluid loop. The natural circulation of fluid is promoted,by relying on gravity to cause the higher density cold fluid to fall andthe lower density heated fluid to rise, causing natural circulationthrough the intermediate coolant loop through a heat exchanger. In somecases, a preferential flow regulator causes a high pressure drop in afirst flow direction and a low pressure drop in a second flow direction.

According to some embodiments, a sodium-cooled nuclear reactor includesa reactor vessel; a reactor core within the reactor vessel; a primaryheat exchanger within the reactor vessel; an intermediate coolant loopconfigured to circulate intermediate coolant through the primary heatexchanger, the intermediate coolant loop having a hot leg and a cold legthrough which intermediate coolant flows; a pump in fluid communicationwith the intermediate coolant loop and configured to circulateintermediate coolant through the intermediate coolant loop; an air heatexchanger disposed along a bypass flowpath between the hot leg and thecold leg of the intermediate coolant loop; and a fluidic diode disposedalong the bypass flowpath between to asymmetrically restrict fluid flowalong the bypass flowpath.

In some cases, the fluidic diode is disposed outside the reactor vessel.The fluidic diode may provide a flow resistance against the pumpcirculating the intermediate coolant across the bypass flowpath. Thatis, when the pump operates, the fluidic diode provides a resistance tothe intermediate coolant from flowing along the bypass flowpath from thecold leg to the hot leg.

The fluidic diode may allow relatively unrestricted fluid flow acrossthe bypass flowpath when the pump is not operating. In other words, whenthe pump is not operating, natural circulation will cause theintermediate fluid to flow through the fluidic diode from the hot leg tothe cold leg with very little flow resistance.

In some instances, the air heat exchanger dumps reactor output heatduring a low-power startup operating condition. For example, the airheat exchanger may dump reactor output heat during a low-power shutdownoperating condition, such as when the reactor output drops below about20% power, or 15% power, or 10% power, or less.

In some examples, the intermediate coolant is liquid sodium, while inother examples, the intermediate coolant is molten salt. Of course,other intermediate coolants are contemplated and can be used with thedisclosed system and configurations.

In some embodiments, a bypass flow pump may be configured to causeintermediate coolant to flow along the bypass flowpath. In some cases,the bypass flow pump is omitted, and natural circulation causes theintermediate coolant to flow along the bypass flowpath.

According to some embodiments, a method of operating a nuclear reactorhaving an intermediate coolant loop includes generating heat in areactor core of a nuclear reactor; causing primary coolant to flowwithin the nuclear reactor through a primary heat exchanger; causing anintermediate coolant to flow through an intermediate coolant loop andthrough the primary heat exchanger; causing the intermediate coolant toflow through a bypass flowpath; and dumping heat, during a period of lowreactor power, from an air heat exchanger disposed along the bypassflowpath in the intermediate coolant loop.

In some examples, the method includes continuing to dump heat from theair heat exchanger disposed along the bypass flowpath until the nuclearreactor reaches a target threshold power output. In other words, aportion of the intermediate coolant may be diverted away from the bypassflowpath as the nuclear reactor exceeds the target threshold poweroutput.

In some embodiments, causing the intermediate coolant to flow through abypass flowpath further includes causing the intermediate coolant toflow through a fluidic diode in fluid communication with the bypassflowpath.

In some instances, the period of low reactor power is less than about 5%reactor power, or less than about 10% of the reactor power, or less thanabout 20% reactor power. In some instances, the intermediate coolant iscaused to dump heat through the air heat exchanger during startup of anuclear reactor. In some instances, the intermediate coolant is causedto dump heat through the air heat exchanger during shutdown of a nuclearreactor.

In some embodiments, causing the intermediate coolant to flow comprisescausing sodium or salt to flow through the intermediate cooling loop.Other suitable coolants may be utilized in the alternative depending onthe reactor configuration and the intermediate loop configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a nuclear reactor intermediatecoolant loop in a steady state operating condition, in accordance withsome embodiments;

FIG. 2 illustrates a schematic representation of a nuclear reactorintermediate coolant loop in a natural circulation operating condition,in accordance with some embodiments;

FIG. 3 illustrates a schematic representation of a nuclear reactorintermediate coolant loop incorporating a fluidic diode in a steadystate operating condition, in accordance with some embodiments; and

FIG. 4 illustrates a schematic representation of a nuclear reactorintermediate coolant loop incorporating a fluidic diode in a naturalcirculation operating condition, in accordance with some embodiments.

DETAILED DESCRIPTION

This disclosure generally relates to a passive apparatus and arrangementfor decay heat removal from a nuclear reactor during shutdown and/orstartup, and include a passive system that can operate throughprincipals of natural circulation.

FIG. 1 illustrates a nuclear reactor intermediate coolant loop in asteady state operating condition. A nuclear reactor 100 of any suitabledesign, will typically include a core within a reactor vessel 104. Thecore will typically contain fuel that generates heat through afissioning process. A primary coolant, which may be any suitablecoolant, is forced through the core 102 to extract heat from thefissioning fuel. After the primary coolant passes through the core, itis routed to a primary heat exchanger 106 where it transfers thermalenergy to an intermediate coolant flowing in an intermediate coolantloop 108.

In the illustrated embodiment, the primary coolant within the reactorvessel may be any suitable coolant, and in some examples is a liquidmetal, such as sodium. The intermediate coolant within the intermediatecoolant loop 108 may likewise be any suitable coolant, but in some casesis sodium. In some examples, the intermediate coolant within theintermediate coolant loop 108 is a salt.

The intermediate coolant loop 108, in many cases, is a closed fluid loopthat includes a hot leg 110, a cold leg 112, and a pump 114. Duringnormal use, the pump 114 pressurizes the intermediate coolant and causesthe coolant within the cold log 108 to flow into the primary heatexchanger 106 and received thermal energy from the primary coolantwithin the reactor 100. The intermediate coolant exits the primary heatexchanger 106 through the hot leg 110 and is transported to anothersystem where the thermal energy is used for a useful purpose. In someembodiments, the thermal energy transferred by the intermediate coolantloop 108 is used to store thermal energy, create steam, or is providedfor other purposes.

In some embodiments, an air heat exchanger 116 is provided between thehot leg 110 and the cold leg 112 along a bypass flowpath 118. In someinstances, the air heat exchanger 116 is elevated, and the force ofgravity inhibits bypass flow from the cold leg 112 directly to the hotleg 110. Bypass flow tends to reduce the temperature of the intermediatecoolant in the hot leg 110 and thus reduces the thermal energy availablefor useful purposes downstream of the bypass flow path 118.

During normal operation of the nuclear reactor 100, the pump 114pressurizes the fluid in the intermediate coolant loop 108 to cause theintermediate coolant to flow through the primary heat exchanger 106.Because the intermediate coolant loop is pressurized, there is anopportunity for the intermediate coolant to flow along the bypass fluidpath 118 in parallel with the flowpath through the primary heatexchanger 106. In some cases, this is an unwanted situation because thebypass flowpath results in mixing of the cold leg with the hot leg andthus reduces the thermal energy of the intermediate coolant in the hotleg. Elevating the air heat exchanger 116 may alleviate some of thebypass flow due to the forces of gravity imparting a flow resistancealong the bypass flowpath 118.

FIG. 2 illustrates a nuclear reactor intermediate coolant loop 108 in areactor shutdown configuration. As illustrated, the pump 114 may bedevoid of power and therefore not pressurizing the intermediate coolantloop 108 to circulate the intermediate coolant. The intermediate coolantmay continue to flow through natural circulation, such as in a directionindicated by arrow 202. In the event of a reactor shutdown, the decayheat from the reactor core needs to be dealt with, and is typicallydealt with by a residual heat removal system, which may be any of anumber of suitable safety systems configured to dump residual heat awayfrom the reactor core. As illustrated, where the pump 114 is notenergized and providing a pumping force, the fluid in the hot leg 110and the cold leg 112 of the intermediate coolant lop 108 may becomestagnant and therefore not carry thermal energy away from the reactorcore, as intended. The air heat exchanger 116 may continue to allownatural circulation flow that provides a fluid pathway for theintermediate coolant that allows the fluid in the hot leg 110 to flowthrough the air heat exchanger 116 and to the cold leg 112 and circulateto continue to provide heat removal from the reactor core 102, even inthe absence of pumping power. Of course, a pump may be provided, such asalong the bypass flowpath 118 to provide forced circulation of theintermediate coolant fluid from the primary heat exchanger 106 to theair heat exchanger 116 to allow decay heat to be dumped to the ambientair. The illustrated embodiment is schematically represented, and inmany cases, the air heat exchanger 116 may be elevated, thus providingnatural circulation by relying on gravity to cause the higher densitycold fluid to fall and the lower density heated fluid to rise, causingnatural circulation through the intermediate coolant loop through theair heat exchanger 116.

In many instances, the bypass flowpath 118 is accessed automatically,even in the absence of power, and does not require any modifications tothe fluid path to cause the intermediate coolant to flow through the airheat exchanger 116. For example, the hot leg 110 and the cold leg 112may provide sufficient flow friction that the bypass flowpath 118becomes the path of least resistance for the fluid flow. In this case,the intermediate coolant with flow from the primary heat exchanger 106,thus withdrawing heat from the reactor core 102, along the hot leg 110to the air heat exchanger 116, to the cold leg 112, and back to theprimary heat exchanger 106, thus receiving thermal energy from thereactor core 102 and dumping it to the ambient air at the air heatexchanger 116.

In some embodiments, there may be a pump provided along the bypassflowpath 118 to further encourage the dumping of decay heat. While theair heat exchanger 116 may be elevated to provide resistance to bypassflow of intermediate fluid from the cold leg 112 to the hot leg 110, insome embodiments, additional resistance to bypass flow may be introducedto further reduce the likelihood of bypass flow.

FIG. 3 illustrates a nuclear reactor intermediate coolant loop 300 witha fluidic diode 302 incorporated into the bypass flowpath 118 to providepreferential flow resistance. During normal full power operation of thenuclear reactor 100, the pump 114 will circulate intermediate fluidthrough the intermediate coolant loop 108. A fluidic diode 302 may beprovided in the bypass flowpath 118, such as between the cold leg 112and the air heat exchanger 116 and be configured to present an increasedflow resistance to inhibit intermediate coolant from entering the airheat exchanger 116 from the cold leg 112.

A fluidic diode 302 is a device that presents increased flow resistanceand therefore limits the fluid flow in one direction, while providingfor a lower resistance to fluid flow in a second direction. A checkvalve could be used for such a configuration. A check valve typicallyhas a moveable component, such as a sealing member, that is moveablebetween an open and a closed position, where the sealing member isbiased in a closed position in response to fluid flow in a firstdirection, and biased in an open position in response to fluid flow in asecond direction. However, check valves may be complex, unreliable, andin some cases, can be fouled or stuck in either an open or closedconfiguration. Accordingly, a fluidic diode 302 can provide many of thesame benefits without the complexity or likelihood of failing.

According to some embodiments, a fluidic diode has no moving parts,rather, differential fluid friction results from the shape of theinternal fluid pathway and/or the size of the inlet and outlet. Theresult is a valve with no moving parts that provides a high resistanceto flow in a first direction and a low resistance to flow in a seconddirection. According to some embodiments, the pressure drop caused bythe fluidic diode in a first direction is on the order of 1.5 times asgreat as the pressure drop caused by the fluidic diode in the seconddirection, or two times as great, or five times as great, or ten timesas great, or more. In some embodiments, the fluidic diode resists flowfrom the cold leg 112 to the hot leg 110 across the bypass flowpath 118.The fluidic diode 302 may allow fluid flow from the hot leg 110 to thecold leg 112 through the air heat exchanger 116 with significantly lessresistance than fluid flow in the opposing direction.

FIG. 4 illustrates a nuclear reactor intermediate coolant loop 300 witha fluidic diode 302 incorporated to provide preferential flowresistance. As illustrated, when the intermediate pump 114 is notpumping to pressurize the intermediate coolant loop 300, theintermediate coolant fluid may flow in the direction indicated by thearrow 202, by natural circulation. In this configuration, fluid from thehot leg 110 will naturally flow through the air heat exchanger 116,through the fluidic diode 302, and to the cold leg 112.

There are situations where it is desirable to vent the reactor generatedheat rather than send the thermal energy to its intended destination,such as a power cycle, or thermal storage. For instance, during reactorstartup, the air heat exchanger 116 may be utilized to dump thegenerated thermal energy until the reactor reaches a threshold operatingpower. For example, during reactor startup, the intermediate coolant mayflow through the air heat exchanger 116 as the reactor ramps up inpower, and in some cases, the air heat exchanger is utilized to dump thereactor generated heat during a low-power operating condition, such as,for example, until the reactor reaches 3% power, or 4%, power, or 5%power, or 7% power, or 8% power, or 10% power or more. As the reactorgradually ramps up in power after startup, the heat transfer cangradually be transferred to the primary heat transport structures andthe air heat exchanger 116 may be substantially idle once the reactorreaches a threshold power output.

Similarly, during reactor shutdown, the air heat exchanger 116 may beused to dump thermal energy and cool the reactor as it is shutting downduring a low-power operating condition. For instance, as the reactorreduces to about 20% power output, the intermediate coolant loop maybegin flowing intermediate coolant through the air heat exchanger 116 todump excess heat. As the reactor continues to reduce in power outputbelow about 20%, the heat transfer duties may be handed off to the airheat exchanger 116 commensurately, so that once the reactor reachesabout 5% power during a shutdown, the air heat exchanger 116 is handling100% of the decay heat. The air heat exchanger 116 can safely handle thedecay heat removal from the reactor during a shutdown in a passive mode,with no moving parts, and no required intervention to activate anysystems, valves, or pumps.

In some embodiments, the air heat exchanger 116 receives thermal energyin response to reactor power output. For example, on startup, the airheat exchanger 116 initially receives 100% of the thermal energyproduced by the reactor until the reactor reaches a threshold poweroutput, such as, for example, about 5% power output. At this point, theair heat exchanger 116 begins gradually passing the thermal energy tothe normal thermal management systems, such as through activation of theintermediate coolant pump 114, until the reactor reaches anotherthreshold power output and the entirety of the thermal energy is passedover to the thermal management systems. In many cases, the fluidic diode302 constrains fluid flow asymmetrically between a first flow directionand a second flow direction, thus inhibiting flow in the first directionwhen the intermediate coolant loop is pressurized and the nuclearreactor is operating at maximum power output, and allowing flow in thesecond direction during low reactor power output scenarios, such asstartup and/or shutdown. In some cases, fluid flow in the seconddirection is substantially unrestricted as compared with fluid flow inthe first direction.

In some examples, the intermediate coolant is sodium and the primaryheat exchanger is a sodium/sodium heat exchanger. In some examples, theintermediate coolant is salt and the primary heat exchanger is asodium/salt heat exchanger. Of course, these configurations rely on anuclear reactor that utilizes sodium as a primary coolant and otherprimary coolants are equally applicable with the disclosed systems andconfigurations.

The disclosure sets forth example embodiments and, as such, is notintended to limit the scope of embodiments of the disclosure and theappended claims in any way. Embodiments have been described above withthe aid of functional building blocks illustrating the implementation ofspecified components, functions, and relationships thereof. Theboundaries of these functional building blocks have been arbitrarilydefined herein for the convenience of the description. Alternateboundaries can be defined to the extent that the specified functions andrelationships thereof are appropriately performed.

The foregoing description of specific embodiments will so fully revealthe general nature of embodiments of the disclosure that others can, byapplying knowledge of those of ordinary skill in the art, readily modifyand/or adapt for various applications such specific embodiments, withoutundue experimentation, without departing from the general concept ofembodiments of the disclosure. Therefore, such adaptation andmodifications are intended to be within the meaning and range ofequivalents of the disclosed embodiments, based on the teaching andguidance presented herein. The phraseology or terminology herein is forthe purpose of description and not of limitation, such that theterminology or phraseology of the specification is to be interpreted bypersons of ordinary skill in the relevant art in light of the teachingsand guidance presented herein.

The breadth and scope of embodiments of the disclosure should not belimited by any of the above-described example embodiments, but should bedefined only in accordance with the following claims and theirequivalents.

Throughout the instant specification, the term “substantially” inreference to a given parameter, property, or condition may mean andinclude to a degree that one of ordinary skill in the art wouldunderstand that the given parameter, property, or condition is met witha small degree of variance, such as within acceptable tolerances. By wayof example, depending on the particular parameter, property, orcondition that is substantially met, the parameter, property, orcondition may be at least approximately 90% met, at least approximately95% met, or even at least approximately 99% met.

Conditional language, such as, among others, “can,” “could,” “might,” or“may,” unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainimplementations could include, while other implementations do notinclude, certain features, elements, and/or operations. Thus, suchconditional language generally is not intended to imply that features,elements, and/or operations are in any way required for one or moreimplementations or that one or more implementations necessarily includelogic for deciding, with or without user input or prompting, whetherthese features, elements, and/or operations are included or are to beperformed in any particular implementation.

The specification and drawings disclose examples of systems, apparatus,devices, and techniques that may allow modules of a nuclear reactor tobe fabricated in a manufacturing facility and shipped to a constructionsite, where the modules can be assembled, thereby greatly reducingon-site fabrication complexity and cost. Further, the systems of thenuclear reactor have been simplified and further promote factoryfabrication in lieu of on-site fabrication.

A person of ordinary skill in the art will recognize that any process ormethod disclosed herein can be modified in many ways. The processparameters and sequence of the steps described and/or illustrated hereinare given by way of example only and can be varied as desired. Forexample, while the steps illustrated and/or described herein may beshown or discussed in a particular order, these steps do not necessarilyneed to be performed in the order illustrated or discussed.

The various exemplary methods described and/or illustrated herein mayalso omit one or more of the steps described or illustrated herein orcomprise additional steps in addition to those disclosed. Further, astep of any method as disclosed herein can be combined with any one ormore steps of any other method as disclosed herein.

It is, of course, not possible to describe every conceivable combinationof elements and/or methods for purposes of describing the variousfeatures of the disclosure, but those of ordinary skill in the artrecognize that many further combinations and permutations of thedisclosed features are possible. Accordingly, various modifications maybe made to the disclosure without departing from the scope or spiritthereof. Further, other embodiments of the disclosure may be apparentfrom consideration of the specification and annexed drawings, andpractice of disclosed embodiments as presented herein. Examples putforward in the specification and annexed drawings should be considered,in all respects, as illustrative and not restrictive. Although specificterms are employed herein, they are used in a generic and descriptivesense only, and not used for purposes of limitation.

Unless otherwise noted, the terms “connected to” and “coupled to” (andtheir derivatives), as used in the specification, are to be construed aspermitting both direct and indirect (i.e., via other elements orcomponents) connection. In addition, the terms “a” or “an,” as used inthe specification, are to be construed as meaning “at least one of.”Finally, for ease of use, the terms “including” and “having” (and theirderivatives), as used in the specification, are interchangeable with andhave the same meaning as the word “comprising.”

From the foregoing, and the accompanying drawings, it will beappreciated that, although specific implementations have been describedherein for purposes of illustration, various modifications may be madewithout deviating from the spirit and scope of the appended claims andthe elements recited therein. In addition, while certain aspects arepresented below in certain claim forms, the inventors contemplate thevarious aspects in any available claim form. For example, while onlysome aspects may currently be recited as being embodied in a particularconfiguration, other aspects may likewise be so embodied. Variousmodifications and changes may be made as would be obvious to a personskilled in the art having the benefit of this disclosure. It is intendedto embrace all such modifications and changes and, accordingly, theabove description is to be regarded in an illustrative rather than arestrictive sense.

What is claimed is:
 1. A sodium-cooled nuclear reactor, comprising: areactor vessel; a reactor core within the reactor vessel; a primary heatexchanger within the reactor vessel; an intermediate coolant loopconfigured to circulate intermediate coolant through the primary heatexchanger, the intermediate coolant loop having a hot leg and a cold legthrough which intermediate coolant flows; a pump in fluid communicationwith the intermediate coolant loop and configured to circulateintermediate coolant through the intermediate coolant loop; an air heatexchanger disposed along a bypass flowpath between the hot leg and thecold leg of the intermediate coolant loop; and a fluidic diode disposedalong the bypass flowpath between to asymmetrically restrict fluid flowalong the bypass flowpath.
 2. The sodium-cooled nuclear reactor as inclaim 1, wherein the fluidic diode is disposed outside the reactorvessel.
 3. The sodium-cooled nuclear reactor as in claim 1, wherein thefluidic diode provides a flow resistance against the pump circulatingthe intermediate coolant across the bypass flowpath.
 4. Thesodium-cooled nuclear reactor as in claim 1, wherein the fluidic diodeallows relatively unrestricted fluid flow across the bypass flowpathwhen the pump is not operating.
 5. The sodium-cooled nuclear reactor asin claim 1, wherein the air heat exchanger dumps reactor output heatduring a low-power startup operating condition.
 6. The sodium-coolednuclear reactor as in claim 1, wherein the air heat exchanger dumpsreactor output heat during a low-power shutdown operating condition. 7.The sodium-cooled nuclear reactor as in claim 1, wherein theintermediate coolant is sodium.
 8. The sodium-cooled nuclear reactor asin claim 1, wherein the intermediate coolant is salt.
 9. Thesodium-cooled nuclear reactor as in claim 1, further comprising a bypassflow pump configured to cause intermediate coolant to flow along thebypass flowpath.
 10. The sodium-cooled nuclear reactor as in claim 1,wherein the fluidic diode provides greater than five times more flowrestriction to fluid flowing in a first direction as compared with fluidflowing in a second direction.
 11. A method of operating a nuclearreactor having an intermediate coolant loop, comprising: generating heatin a reactor core of a nuclear reactor; causing primary coolant to flowwithin the nuclear reactor through a primary heat exchanger; causing anintermediate coolant to flow through an intermediate coolant loop andthrough the primary heat exchanger; causing the intermediate coolant toflow through a bypass flowpath; and dumping heat, during a period of lowreactor power, from an air heat exchanger disposed along the bypassflowpath in the intermediate coolant loop.
 12. The method of operating anuclear reactor as in claim 11, further comprising continuing to dumpheat from the air heat exchanger disposed along the bypass flowpathuntil the nuclear reactor reaches a target threshold power output. 13.The method of operating a nuclear reactor as in claim 12, furthercomprising diverting a portion of the intermediate coolant away from thebypass flowpath as the nuclear reactor exceeds the target thresholdpower output.
 14. The method of operating a nuclear reactor as in claim11, wherein causing the intermediate coolant to flow through a bypassflowpath further comprises causing the intermediate coolant to flowthrough a fluidic diode in fluid communication with the bypass flowpath.15. The method of operating a nuclear reactor as in claim 11, whereinthe period of low reactor power is less than about 5% reactor power. 16.The method of operating a nuclear reactor as in claim 11, wherein theperiod of low reactor power is less than about 20% reactor power. 17.The method of operating a nuclear reactor as in claim 11, wherein theperiod of low reactor power occurs as the nuclear reactor is shuttingdown.
 18. The method of operating a nuclear reactor as in claim 17,wherein the step of dumping heat comprises dumping decay heat from thenuclear reactor during a reactor shut down cycle.
 19. The method ofoperating a nuclear reactor as in claim 11, wherein causing theintermediate coolant to flow comprises causing sodium to flow throughthe intermediate cooling loop.
 20. The method of operating a nuclearreactor as in claim 11, wherein causing the intermediate coolant to flowcomprises causing salt to flow through the intermediate cooling loop.